Current Research in EEPS Seminar: Dr. Justin Mankin, Dartmouth College
Will plants make the world wetter or drier?
A long-held finding suggests that plants, through their influence on surface resistance to evapotranspiration (ET), will generate more global-scale runoff under increasing atmospheric CO2 concentrations ([CO2]). Recent studies using models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) ostensibly reaffirm this result, further suggesting that plants will ameliorate the dire reductions in water availability projected by other studies that use aridity metrics. I complicate this narrative, showing that projected plant responses directly reduce future runoff across vast swaths of North America, Europe, and Asia because bulk canopy water demands increase with additional vegetation growth and longer and warmer growing seasons. These runoff declines occur despite increased surface resistance to ET and vegetation total water use efficiency (WUE), even in regions with increasing or unchanging precipitation. Crucially, my constraining the large uncertainty in the CMIP5 ensemble with regional-scale observations of ET partitioning strengthens these results. What is clear is that vegetation has a large, unresolved role in shaping future regional freshwater availability, and if we are to believe the models, will not ubiquitously ameliorate future warming-driven surface drying.